As a technique for visualizing a two-dimensional distribution of a high-frequency electric field in real time, the live electro-optic imaging technique, which has been invented by NICT (National Institute of Information and Communications Technology) (see NPL 1) is known. This is based on the electromagnetic field high speed imaging apparatus disclosed in PTL 1 and improvements that follow.
The live electrooptic imaging camera mentioned above is a device by which an electric field distribution in microwave bands up to 100 GHz is imaged and displayed in real time by using both super parallelism and high speed of light through an electrooptic effect.
If not restricted to “real-time” imaging, various types of techniques for imaging the distributions of electric fields have been proposed and implemented. So far, electric fields surrounding various circuits and devices have been imaged and used to analyze the circuits and devices, but spatial scanning has conventionally been a time-consuming process. Even under such a constraint, continuous efforts have been made to visualize electric fields and there have been high expectations for the development of “real-time” imaging.
FIG. 1(a) is a block diagram of the mentioned above and FIG. 1(b) illustrates a driver unit of the live electrooptic imaging camera. According to PTL 1, the driver unit includes an oscillation fLO portion for a flashing light source and an oscillation fRF portion for a sample. In the live electrooptic imaging camera, when the flashing light source irradiates an electro-optic (EO) crystal plate (probe in PLT1) with polarized light (illuminating light) which is amplitude-modulated at frequency fLO, the electric field of a radio wave of frequency fRF emitted from the sample (DUT) changes the birefringence properties of the EO crystal state. This gives a change that reflects the change in birefringence at each position to the polarization state of the illuminating light. The polarized light with the polarization state changed as described above is selected by an optical system including an analyzer and the corresponding two-dimensional distribution is imaged by an image sensor. Thus, for detected light containing a difference frequency component (denoted by Δf (=|fLO−fRF|) in PTL 1) between the modulation frequency (fLO) of the illuminating light and the frequency (fRF) of the electromagnetic field emitted from the sample, signals of respective pixels of the image sensor are obtained in parallel. The signals are sequentially output on a pixel-by-pixel basis, subjected to computation by an image processing device, and then displayed by an image display device.
In the electric field imaging technique using this live electrooptic imaging camera, a high-frequency electrical signal to be imaged is synchronized with the sampling by an internal high-speed CMOS image sensor (IS) of the imaging system. This is to avoid unstable display or disappearance of the result of simultaneous frequency conversion performed in the pixels of the image sensor.
To achieve this synchronization, as illustrated in FIG. 1(b), signal sources capable of synchronizing with a reference signal from an external signal source have been used for signal sources for frequency fLO and frequency fRF. Therefore, for the signal sources, the IS serves as a master and a DUT operation serves as a slave. This means that electric field imaging has been conventionally done by a system with a synchronization mode and a master mode.
That is, as illustrated in FIG. 1(b), the signal source for frequency fLO is also synchronized with an IS reference signal. This is because the difference frequency component Δf between the signal for frequency fLO and the RF signal needs to be synchronized with the timing of IS sampling. The band of the difference frequency component Δf is limited to a video frame rate (typically 10 Hz) or lower. To achieve this, if the instantaneous frequency of the RF signal is changed by modulation or data superimposition, the signal for frequency fLO needs to follow the RF signal so as to maintain the frequency difference Δf between the signal for frequency fLO and the RF signal. This gives a limitation to both the DUT and the LO signal source.
As described above, electromagnetic waves that can be observed by the conventional live electrooptic imaging camera system have been very limited. For example, samples have been limited to those capable of synchronizing with an external signal source (primarily a synthesizer) or those capable of synchronizing with an IS clock. Generally, commercially available products or high-frequency modules used in the products require modification, such as addition of a circuit having a synchronization function or installation of additional wiring for signal input and output. There have rarely been products or modules that can serve as the above-described samples without being modified.
Signals used here are synchronized with a reference signal from the IS as described above. For the synchronization and the necessity to follow the RF signal described above, frequency synthesizers have conventionally been used as the signal sources for frequency fLO and frequency fRF. In this case, however, their bandwidths are 1 Hz or less and the RF signal has a very narrow band. Generally, in the case of an actual module including a free-running signal source and a device using the module RF signal exhibits considerable changes and have wide bandwidths. Although the degree of changes differs from one high-frequency device to another, it is desirable that a bandwidth suitable for the sample be achievable.